Before starting any experiment, make sure that the Voltage Source and the Clock is calibrated. You may use an oscilloscope for doing this. If you need any help on this, contact the TA during the lab session. Since the lab is used for many other courses, the voltage source could have been tuned to supply "15V". Connect the voltage supply across the oscilloscope and adjust it to give an output of exactly "5V".
Many students commit the mistake of not supplying power to the ICs. Check to make sure that you have connected the power supply to the appropriate pins. (For a 14-pin 7400 Device, the 7th pin should be connected to the "GND" and the 14th pin to "VCC" ("5V") ). Click here to find out how to connect the power supply for a standard 7400 device.
Often, the digital circuits become large and difficult to debug. It always helps to make a neat schematic of the circuit diagram. You should also plan where to plug the different ICs and how you intend to make the connections. This will make the debugging job easier.
Again, this makes the debugging job easier. Lab experiments are always done in groups of two. Ideally, one student should make the connections on the protoboard while the other should verify them.